The present invention relates to Rxe2x80x94Fexe2x80x94B type rare earth magnets and alloy powder for such magnets, and methods for producing such magnets and alloy powder.
Rare earth sintered magnets are produced by pulverizing an alloy for rare earth magnets to form alloy powder, compacting the alloy powder, and subjecting the alloy powder to sintering and aging. Presently, as the rare earth sintered magnets, samarium-cobalt magnets and rare earth-iron-boron magnets, are extensively used in various fields. In particular, rare earth-iron-boron magnets (hereinafter, referred to as xe2x80x9cRxe2x80x94Fexe2x80x94B type magnetsxe2x80x9d, where R is any rare earth element and/or Y, Fe is iron, and B is boron), which exhibit the highest magnetic energy product among a variety of magnets and have a comparatively low cost, have been extensively applied to various types of electronic equipment. Note that a transition metal element such as Co may be substituted for a portion of Fe and C (carbon) may be substituted for a portion of B (boron) in such Rxe2x80x94Fexe2x80x94B type magnets.
Powder of the material alloy for Rxe2x80x94Fexe2x80x94B type rare earth magnets may be produced by a method including a first pulverization process for coarsely pulverizing the material alloy and a second pulverization process for finely pulverizing the material alloy. In general, in the first pulverization process, the material alloy is coarsely pulverized to an average particle size that is several hundred micrometers or less using a hydrogen embrittlement apparatus. In the second pulverization process, the coarsely pulverized alloy (coarsely pulverized powder) is finely pulverized to an average particle size that is several micrometers with a jet mill or other suitable apparatus.
The material alloy can be produced by methods that are generally classified into two types. The first type of method is an ingot casting method where a molten material alloy is poured into a mold and cooled comparatively slowly. The second type of method is a rapid cooling method, typified by a strip casting method and a centrifugal casting method, where a molten material alloy is put into contact with a single chill roll, twin chill rolls, a rotary chill disk, a rotary cylindrical chill mold, or other similar device, to be rapidly cooled thereby producing a solidified alloy that is thinner than an ingot cast alloy.
In the rapid cooling method, the molten alloy is cooled at a rate in the range of 102xc2x0C./sec to 104xc2x0C./sec. The resultant alloy produced by the rapid cooling method has a thickness in the range of 0.03 mm to 10 mm. The molten alloy starts solidifying at the surface that comes into contact with a chill roll. From the roll contact surface, crystal grows in the thickness direction into the shape of pillars or needles. The resultant rapidly solidified alloy therefore has a fine crystal structure including portions of a R2T14B crystal phase having a size in the range of 0.1 xcexcm to 100 xcexcm in the minor-axis direction and in the range of 5 xcexcm to 500 xcexcm in the major-axis direction, and portions of an R-rich phase dispersed at grain boundaries of the R2T14B crystal phase portions. The R-rich phase is a nonmagnetic phase in which the concentration of any rare earth element R is relatively high, and has a thickness (which corresponds to the width of the grain boundaries) of 10 xcexcm or less.
Because the rapidly solidified alloy is cooled in a relatively short time compared with an ingot alloy produced by a conventional ingot casting method, the alloy has a fine structure and small grain size. In addition, with finely dispersed crystal grains, the area of grain boundaries is wide, and thus the R-rich phase spreads thinly over the grain boundaries. This results in good dispersion of the R-rich phase.
When a rare earth alloy (especially a rapidly solidified alloy) is coarsely pulverized in a hydrogen embrittlement process where the rare earth alloy first occludes hydrogen (this way of pulverization is herein referred to as xe2x80x9chydrogen pulverizationxe2x80x9d), the R-rich phase portions existing at grain boundaries react with hydrogen and expand. Therefore, the alloy tends to start cracking from the R-rich phase portions (grain boundary portions). As a result, the R-rich phase tends to be exposed on the surfaces of particles of the rare earth alloy powder obtained by the hydrogen pulverization. In addition, in the case of a rapidly solidified alloy, where the R-rich phase portions are fine and highly dispersed, the R-rich phase particularly tends to be exposed on the surfaces of the hydrogen-pulverized powder. Such an R-rich phase that exists in the powder particle plays an important role during a sintering process of a powder compact. During the sintering process, the R-rich phase melts earlier than R2T14B crystal phase to form a liquid phase which is needed for sintering the powder compact.
Based on experiments conducted by the present inventors, when the coarsely pulverized powder in the above-described state is finely pulverized with a jet mill or other suitable apparatus, R-rich super-fine powder (fine powder having a particle size of 1 xcexcm or less) is produced. Such R-rich super-fine powder particles oxidize very easily compared with other powder particles (having a relatively large particle size) that contain a relatively smaller amount of R. Therefore, if a sintered magnet is produced from the resultant finely pulverized powder without removing such R-rich super-fine powder, oxidation of the rare earth element rapidly proceeds during the manufacturing process steps. The rare earth element R is thus consumed by reacting with oxygen, and as a result, the production amount of the R2T14B crystal phase as the major phase significantly decreases. This results in a decrease in the coercive force and remanent flux density of the resultant magnet and deterioration of the squareness of the demagnetization curve, which is the second quadrant curve of the hysteresis loop.
In order to prevent oxidation of the R-rich finely pulverized powder, the entire process from pulverizing through sintering may ideally be performed in an inert atmosphere. It is however very difficult to realize this environment in a mass-production scale in production facilities.
A method for solving the above-described problem has been proposed, where fine pulverization is performed in an inert atmosphere containing a trace amount of oxygen, to intentionally coat the surfaces of finely pulverized powder particles with a thin oxide film to thereby suppress fast oxidation of the powder particles in the atmosphere.
However, the method described in the preceding paragraph causes a problem as follows when the powder particle size is simply reduced for the purpose of enhancing the coercive force. When the particle size is reduced, the total surface area of particles existing in a given weight of powder increases. This increases the total oxygen amount adsorbed to the surfaces of the powder particles, and as a result, the oxygen concentration of the resultant sintered magnet becomes significantly high. Since oxygen contained in the sintered magnet reacts with the rare earth element R, the amount of the produced R2T14B crystal phase as the major phase is significantly reduced. As a result, the coercive force decreases contrary to the original purpose.
In general, in order to enhance the coercive force, it is considered necessary to reduce the grain size of the R2T14B crystal phase as the major phase to a size closer to the mono-domain grain size (about 0.5 xcexcm). However, the surfaces of the powder particles must be thinly oxidized to avoid the risk of ignition, and this results in decreasing the coercive force, as described above. Therefore, for enhancing the coercive force, simply reducing the powder particle size is not enough. As countermeasures, an expensive rare element such as Dy and Tb that are effective in enhancing the coercive force may be added.
However, addition of such an expensive rare element raises the price of the magnet, and thus may threaten stable supply of magnets. There are therefore strong demands for providing rare earth magnets that exhibit an increased coercive force but do not contain expensive rare elements such as Dy.
In order to solve the problems described above, preferred embodiments of the present invention provide a method for manufacturing a Rxe2x80x94Fexe2x80x94B type rare earth magnet that greatly increases the coercive force thereof while avoiding occurrence of oxidation/ignition due to contact with the atmosphere, also provide a high-performance Rxe2x80x94Fexe2x80x94B type rare earth magnet manufactured by such a novel method.
According to a preferred embodiment of the present invention, a method for manufacturing Rxe2x80x94Fexe2x80x94B type rare earth magnets includes the steps of preparing alloy powder for Rxe2x80x94Fexe2x80x94B type rare earth magnets including particles having a size in a range of about 2.0 xcexcm to about 5.0 xcexcm as measured by a light scattering method using a Fraunhofer forward scattering in a proportion of about 45 vol. % or more and particles having a size larger than about 10 xcexcm in a proportion of less than about 1 vol. %; compacting the powder to produce a compact; and sintering the compact.
Preferably, in the step of sintering, a sintered magnet having an average crystal grain size in a range of about 5 xcexcm to about 7.5 xcexcm is produced.
The concentration of oxygen contained in the sintered magnet is preferably adjusted to be in a range of about 2.2 at. % to about 3.0 at. %.
Preferably, the alloy powder for Rxe2x80x94Fexe2x80x94B type rare earth magnets includes substantially no Dy.
In another preferred embodiment of the present invention, the step of preparing alloy powder for Rxe2x80x94Fexe2x80x94B type rare earth magnets includes a first pulverization step of coarsely pulverizing a material alloy for rare earth magnets produced by a rapidly cooling method and a second pulverization step of finely pulverizing the material alloy, wherein in the second pulverization step, the material alloy for Rxe2x80x94Fexe2x80x94B type rare earth magnets is pulverized in a chamber of a pulverizer filled with inert gas containing an oxidizing gas.
Preferably, a classifier is connected to follow the pulverizer for classifying powder coming out from the pulverizer.
In another preferred embodiment of the present invention, the material alloy for rare earth magnets is obtained by cooling a molten material alloy at a cooling rate in a range of about 102xc2x0C./sec to about 104xc2x0C./sec.
The molten material alloy is preferably cooled by a strip casting method.
The Rxe2x80x94Fexe2x80x94B type rare earth magnet of various preferred embodiments of the present invention has an average crystal grain size in a range of about 5 xcexcm to about 7.5 xcexcm, and an oxygen concentration in a range of about 2.2 at. % to about 3.0 at. %.
Preferably, alloy powder as a material of the Rxe2x80x94Fexe2x80x94B type rare earth magnet includes substantially no Dy.
Other features, processes, steps, characteristic and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings.